Define WIN32_LEAN_AND_MEAN in the build system, not in the code.

This avoids a conflict with the Chromium build system, which
defines WIN32_LEAN_AND_MEAN with a different value.

BUG=crbug.com/453196

Change-Id: Ia15ec7c20325c1961af4f32e5208266e5f846f35
Reviewed-on: https://boringssl-review.googlesource.com/3150
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index b67c617..7451c4b 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -10,6 +10,7 @@
   set(CMAKE_C_FLAGS "/wd4267")
   set(CMAKE_CXX_FLAGS "/wd4267")
   add_definitions(-D_HAS_EXCEPTIONS=0)
+  add_definitions(-DWIN32_LEAN_AND_MEAN)
 endif()
 
 if((CMAKE_COMPILER_IS_GNUCXX AND CMAKE_C_COMPILER_VERSION VERSION_GREATER "4.5.99") OR